When ARCO suspended underground pumping operations in 1982, groundwater levels on the Butte Hill began to rise. Nineteen months later the water level in the underground workings and surrounding bedrock reached the bottom of the Pit, allowing bedrock groundwater to start filling the Pit void.
Prior to that time alluvial groundwater seeped into the Pit from the east and south walls, beginning to fill the Pit lake. ARCO also diverted water from its mining operations (leach pad water, Continental Pit, Horseshoe Bend, etc.) into the Pit following the 1983 shutdown of their entire Butte operations.
When Montana Resources began operations in 1986 a number of these surface water sources were diverted away from the Pit, however, the Horseshoe Bend water continued to flow into the Pit until April 1996 when it was incorporated in Montana Resource’s mining operations for treatment and disposal in the Yankee Doodle Tailings Dam.
When Montana Resources suspended mining operations from 2000 through 2003, about 7.5 billion gallons of water, or an average of 6 million gallons per day, went into the Pit. Of this total, an average of 3.4 million gallons per day came from rising groundwater flows in the underground mine workings and surface stormwater flow. An average of 2.6 million gallons per day came from the Horseshoe Bend drainage. Montana Resources also diverted water from the Continental Pit into the Berkeley Pit for containment during their suspension.
Since the Horseshoe Bend Water Treatment Plant began operating in 2003, water flows from the Horseshoe Bend drainage have been diverted to the treatment plant. After treatment, this Horseshoe Bend water is entirely recycled or consumed in mining operations, or, in other words, no water is discharged off of the site.
About 2.6 million gallons per day from groundwater and stormwater still flow into the Pit, contributing to the rising level there. Eventually, when the water level approaches the Critical Level of 5,410 feet above sea level, water will be pumped from the Berkeley Pit and treated at the Horseshoe Bend facility. Present projections put this date around 2023. Having the plant in place provides assurance that the capability to manage Berkeley Pit water levels is there when it becomes necessary to treat Pit water.